Repairs by fly ash concrete to extend service life of chloride-exposed concrete structures considering environmental impacts

• Repairs by fly ash concrete on chloride-exposed concrete structures are assessed.
• Time-dependent model of surface chloride and diffusion coefficient is used.
• Model of CO2 emission from concrete production and repair processing is developed.
• Present value of carbon price transformed from the amount of CO2 is predicted.
• Threshold ratio of diffusion coefficient of original to repair concrete is also defined.

This study proposes a quantitative method to assess the corrosion-free service life and the environmental impact in terms of CO2 and carbon price due to repairs by replacing cover concrete with fly ash concrete on chloride-exposed concrete structures. The study takes advantage of the Crank–Nicolson based finite difference approach to simplify the assessment. Using the approach, the service life and the repair time for corrosion-free condition of concrete structures can be predicted. At the time of repairs, the CO2 occurs due to concrete production and replacement processing, and can be assessed using a CO2 emission model developed here. And, the amount of CO2 is transformed into the carbon price. From the study, it can be concluded that the increase of the amount of fly ash in repair concrete by 15% causes the reduction of the cumulative CO2 and carbon price by as high as 58% and 41%, respectively. The ratio of the diffusion coefficient of original concrete to that of repair concrete can be calculated, and its threshold ratio is defined. If the ratio is larger than the threshold ratio, deeper depth of repairs causes shorter extension of corrosion-free period. Furthermore, the threshold ratio apparently decreases with repair depth.